Complex electrical circuits and systems may involve the provision of a plurality of power supplies. Each power supply may operate at a different voltage and/or supply a different load. It is advantageous during the powering up and powering down of such a complex system that the power supplies can be switched on and off in a predetermined sequence in order to ensure reliable and consistent operation of the electrical system. It is also advantageous to be able to monitor the continued operation of the various power supplies in order to determine that they are well behaved, and in the event of one or more supplies becoming faulty, to be able to take an appropriate action in order to ensure the well being or continued operation of the system served by the various power supplies, or alternatively to shut it down in a manner liable to cause the least disruption or damage to that system.